Soft ophthalmic lenses for placement on the cornea or within the eye, such as contact lenses or soft intraocular lenses, can be made by a variety of techniques. Contact lenses can be made by spin casting a prepolymer material in a rotating mold then polymerizing the material so shaped. Another method used to manufacture both contact lenses and intraocular lenses is precision lathing of a piece of material which is then polished and used as a lens.
Recently the molding of soft contact lenses and soft intraocular lenses has come into favor. This technique has the advantages of repeatability and speed that compares favorably with the prior methods of manufacturing lenses. Techniques for successfully molding such lenses can be found in U.S. Pat. Nos. 4,495,313 and 4,889,664. These patents specifically described the use of a diluent, a material which substitutes for water during the molding process, and which is replaced with water after the molding has been completed. The advantage of this technique is that the optical properties, size and shape of the lens thus made does not change as radically as with methods that do not utilize such diluent.
The molds used for forming such lenses are typically made of polystyrene and are described in U.S. Pat. Nos. 4,565,348 and 4,640,489. Although such techniques described therein are successful in making molded ophthalmic lenses, the most frequently encountered defect of such lenses is in the edge portion.
It has been theorized that many of these defects are caused by the fact that in making the lens an excess amount of prepolymer is placed in the female portion of the mold cavity and when the male portion is mated therewith, the excess prepolymer is displaced from the cavity and forms a ring about the periphery of the mated pair of mold portions.
Upon the separation of the two-mold portions, parts of the edge of the polymerized lens may still be attached to the peripheral flashing of now polymerized excess material causing edge irregularities when the two are separated.
This phenomenon is easily understood if one considers that a plane is defined by three points and that any deviation from a perfectly planar mating edge can cause a gap between the male and female portions of the mold.
Some manufacturers implement an edge lathing operation subsequent to the manufacturing of the lens in order to obtain a smooth edge. While polishing is possible with many hydrophilic materials in their dry state, this involves additional processing steps. Furthermore, some lens materials such as silicon, monomer/diluent mixtures and fluoropolymers are not compatible with being polished due to their physical properties.
It is the object therefore of the present invention, to present an ophthalmic lens mold which produces a lens free of edge defects.
This object is accomplished by providing a mold that forms a region of prepolymer discontinuity between the lens and the flashing.
The mold portions of the present invention contain on one portion a knife edge that makes line contact with the other portion and further contain a circumferential lens bushing peripherally exterior to the knife edge such that when the two portions are forced together the knife edge deformably penetrates the mold portion with which it is in contact and the rim bushing makes contact with the opposite mold portion.